High voltage diode with reduced substrate injection

ABSTRACT

A high voltage diode in which the n-type cathode is surrounded by an uncontacted heavily doped n-type ring to reflect injected holes back into the cathode region for recombination or collection is disclosed. The dopant density in the heavily doped n-type ring is preferably 100 to 10,000 times the dopant density in the cathode. The heavily doped n-type region will typically connect to an n-type buried layer under the cathode. The heavily doped n-type ring is optimally positioned at least one hole diffusion length from cathode contacts. The disclosed high voltage diode may be integrated into an integrated circuit without adding process steps.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuits. Moreparticularly, this invention relates to high voltage diodes inintegrated circuits.

BACKGROUND OF THE INVENTION

High voltage diodes, which are able to operate at greater than 50 voltsreverse bias without breaking down, are often included in integratedcircuits (ICs). During operation under forward bias, a high voltagediode may undesirably inject a significant current density of majoritycarriers into the substrate of the IC, interfering with operation ofadjacent components in the IC.

SUMMARY OF THE INVENTION

This Summary is provided to comply with 37 C.F.R. §1.73, requiring asummary of the invention briefly indicating the nature and substance ofthe invention. It is submitted with the understanding that it will notbe used to interpret or limit the scope or meaning of the claims.

The instant invention provides a high voltage diode in an integratedcircuit (IC) formed of a p-type anode inside an n-type cathode and anuncontacted n-type diffused ring region surrounding the cathode, whichhas a higher dopant density than the cathode. The dopant density in theuncontacted n-type diffused ring region is preferably 100 to 10,000times the dopant density in the n-type cathode.

An advantage of the instant invention is a portion of injected holecurrent from the anode is reflected back to the deep n-well cathode bythe uncontacted n-type diffused ring region, thus desirably reducing theamount of injected hole current that diffuses to adjacent components inthe IC.

DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1A through FIG. 1H are cross-sections of an IC containing a highvoltage diode formed according to a first embodiment of the instantinvention, shown in successive stages of fabrication.

FIG. 2 is a cross-section of an IC containing a high voltage diode withmultiple anode fingers formed according to a second embodiment of theinstant invention.

DETAILED DESCRIPTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the invention.Several aspects of the invention are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide a full understanding of the invention. One skilled in therelevant art, however, will readily recognize that the invention can bepracticed without one or more of the specific details or with othermethods. In other instances, well-known structures or operations are notshown in detail to avoid obscuring the invention. The present inventionis not limited by the illustrated ordering of acts or events, as someacts may occur in different orders and/or concurrently with other actsor events. Furthermore, not all illustrated acts or events are requiredto implement a methodology in accordance with the present invention.

The instant invention provides a high voltage diode in an integratedcircuit (IC) formed of a p-type anode inside a deep n-well cathode whichincludes an uncontacted n-type diffused ring region with a higher dopantdensity than the deep n-well cathode on a lateral boundary of the deepn-well cathode. The uncontacted n-type ring reflects a portion ofinjected hole current from the anode back to the deep n-well cathode,thus desirably reducing the amount of injected hole current thatdiffuses to adjacent components in the IC. An advantage of the instantinvention is that the floating n-type diffused ring may be integratedinto the IC without adding process cost or complexity.

FIG. 1A through FIG. 1H are cross-sections of an IC containing a highvoltage diode formed according to a first embodiment of the instantinvention, shown in successive stages of fabrication. Referring to FIG.1A, the IC (100) is formed on a p-type substrate (102), typically asingle crystal silicon wafer, commonly with an electrical resistivitybetween 0.001 and 1 ohm-cm. A p-type epitaxial layer (104) is formed ona top surface of the substrate (102), typically by known vapor phaseepitaxial growth methods. The p-type epitaxial layer (104) is typicallybetween 3 and 9 microns thick, and typically has an electricalresistivity between 1 and 100 ohm-cm. An n-type buried layer (106) isformed at an interface between the substrate (102) and the p-typeepitaxial layer (104) in a region for the inventive high voltage diodeby known processes, including ion implantation of a first set of n-typedopants, typically antimony, but possibly including arsenic, at a totaldose between 1·10¹⁴ and 1·10¹⁷ atoms/cm², through an n-type buried layerimplant mask into a region at the top surface of the substrate (102)defined for the n-type buried layer before the p-type epitaxial layer(104) is formed, followed by a thermal operation which repairs damage toa crystal lattice of the substrate (102), and followed by growth of thep-type epitaxial layer (104). The n-type buried layer (106) commonly hasa thickness of 1 to 3 microns and a dopant density between 5·10¹⁷ and3·10²⁰ atoms/cm², resulting in a sheet resistivity between 1 and 100ohms/square.

FIG. 1B depicts the IC (100) after formation of an uncontacted n-typediffused ring region (108). In the instant embodiment, the uncontactedn-type diffused ring region (108) is formed of deep n-type diffusedregions (108), commonly known as sinkers, in the p-type epitaxial layer(104) adjacent to a lateral boundary of the n-type buried layer (106),typically by ion implanting a second set of n-type dopants, includingphosphorus, and possibly arsenic, at a total dose between 1·10¹⁵ and1·10¹⁷ atoms/cm², at one or more energies between 50 and 500 keV,followed by an n-sinker drive operation in which the IC (100) is heatedhigher than 1000 C for longer than 60 minutes, resulting in an n-typeregion with an average dopant density between 2·10¹⁸ and 2·10²⁰atoms/cm³, extending from a top surface of the substrate (102) to then-type buried layer (106). The n-type sinker regions (108) are joined atlocations out of the plane of FIG. 1B, so as to laterally surround andelectrically isolate a region of the p-type epitaxial layer (104) overthe n-type buried layer (106). Other methods of forming the uncontactedn-type diffused ring region are within the scope of the instantembodiment.

FIG. 1C depicts the IC (100) during an ion implantation operation toform a deep n-well cathode. A deep n-well cathode photoresist pattern(110) is formed on the top surface of the p-type epitaxial layer (104)using known photolithographic methods to define a region for a deepn-well cathode implant. A third set of n-type dopants (112), includingphosphorus and arsenic, and possibly antimony, are ion implanted at atotal dose between 1·10¹² and 3·10¹³ atoms/cm², at one or more energiesbetween 50 and 3000 keV, into the p-type epitaxial layer (104) in theregion defined for the deep n-well cathode implant to form a deep n-wellcathode implanted region (114). The third set of n-type dopants (112) isblocked from the p-type epitaxial layer (104) outside the region definedfor the deep n-well cathode implant by the deep n-well cathodephotoresist pattern (110). The deep n-well cathode photoresist pattern(110) is removed after the deep n-well cathode implant operation,commonly by exposing the IC (100) to an oxygen containing plasma,followed by a wet cleanup to remove any organic residue from the topsurface of the p-type epitaxial layer (104).

FIG. 1D depicts the IC (100) after a deep n-well cathode drive operationwhich diffuses and activates the third set of n-type dopants in the deepn-well cathode implanted region throughout the region of the p-typeepitaxial layer (104) over the n-type buried layer (106) to form a deepn-well cathode (116). The deep n-well cathode drive operation iscommonly performed at a temperature higher than 1000 C for longer than60 minutes. The deep n-well cathode (116) has an average dopant densitybetween 3·10¹⁵ and 2·10¹⁷ atoms/cm³, extending from a top surface of thep-type epitaxial layer (104) to the n-type buried layer (106).

It is common to perform the n-sinker drive operation, if performed, andthe deep n-well cathode drive operation as one operation. It is withinthe scope of the instant invention to form the n-type buried layer(106), the uncontacted n-type diffused ring region (108) and the deepn-well cathode (116) by any means which produces the configurationdescribed above in reference to FIG. 1A through FIG. 1D, such that theaverage dopant density in the uncontacted n-type diffused ring region(108) is 100 to 10,000 times higher than the average dopant density inthe deep n-well cathode (116).

FIG. 1E depicts the IC (100) after formation of field oxide elements, attop surfaces of the p-type epitaxial layer (104) and deep n-well cathode(116), typically of silicon dioxide between 250 and 600 nanometersthick, commonly by shallow trench isolation (STI) or local oxidation ofsilicon (LOCOS) processes. In STI processes, silicon dioxide may bedeposited by high density plasma (HDP) or high aspect ratio process(HARP). A first set of field oxide elements (118) is formed over thedeep n-well cathode (116) to electrically isolate an anode region fromcathode regions of the high voltage diode at the top surface of the deepn-well cathode (116). An optional second set of field oxide elements(120) may be formed over an outer region of the deep n-well cathode(116), and may optionally extend over the uncontacted n-type diffusedring region (108).

FIG. 1F depicts the IC (100) after formation of a shallow p-well anode(122) in the deep n-well cathode (116) under an opening in the first setof field oxide elements (118), typically by ion implanting a first setof p-type dopants, including boron and possibly gallium and/or indium,at doses from 1·10¹¹ to 1·10¹⁴ atoms/cm², into a region defined for ashallow p-well. A p-well photoresist pattern, not shown in FIG. 1E forclarity, is commonly used to block the first set of p-type dopants fromareas outside the p-well region. The shallow p-well anode (122) extendsfrom a top surface of the deep n-well cathode (116) to a depth typically250 to 1500 nanometers below a bottom surface of the field oxideelements (122). The ion implantation process to form the shallow p-wellanode (122) may include additional steps to implant additional p-typedopants at shallower depths for purposes of improving n-channel metaloxide semiconductor (NMOS) transistor performance, such as thresholdadjustment, leakage current reduction and suppression of parasiticbipolar operation.

FIG. 1G depicts the IC (100) after formation of a p-type anode contactregion (124) at a top surface of the shallow p-well anode (122) andn-type cathode contact regions (126) at the top surface of the deepn-well cathode (116). The p-type anode contact region (124) is typicallyformed by ion implanting a second set of p-type dopants, includingboron, commonly in the form BF₂, and possibly gallium and/or indium, ata total dose between 3·10¹³ and 1·10¹⁶ atoms/cm², into areas defined forthe p-type anode contact region (124). An anode contact photoresistpattern, not shown in FIG. 1G for clarity, is commonly used to block thesecond set of p-type dopants from areas outside the anode contactregion. The p-type anode contact region (124) typically extends from thetop surface of the shallow p-well anode (122) to a depth between 50 and500 nanometers. Similarly, the n-type cathode contact regions (126) aretypically formed by ion implanting a fourth set of n-type dopants,including phosphorus and arsenic, and possibly antimony, at a total dosebetween 3·10¹³ and 1·10¹⁶ atoms/cm², into areas defined for the n-typecathode contact regions (126). A cathode contact photoresist pattern,not shown in FIG. 1G for clarity, is commonly used to block the fourthset of n-type dopants from areas outside the cathode contact regions.The n-type cathode contact regions (126) typically extend from the topsurface of the deep n-well cathode (116) to a depth between 50 and 500nanometers. It is within the scope of the instant invention to form thep-type anode contact region (124) and the n-type cathode contact regions(126) in any order.

A lateral separation between the shallow p-well anode (122) and then-type cathode contact regions (126) is typically established by amaximum operating voltage of the high voltage diode. For example, a highvoltage diode that is designed to operate at 80 volts may have a lateralseparation of 3 to 4 microns between the shallow p-well anode (122) andthe n-type cathode contact regions (126). A lateral separation betweenthe n-type cathode contact regions (126) and the uncontacted n-typediffused ring region (108) is preferably greater than a diffusion lengthof positive charge carriers, commonly known as holes, in the deep n-wellcathode (116), which is commonly greater than 5 microns.

FIG. 1H depicts the IC (100) after formation of a first level ofinterconnect elements. A pre-metal dielectric layer (PMD) (128),typically a dielectric layer stack including a silicon nitride orsilicon dioxide PMD liner 10 to 100 nanometers thick deposited by plasmaenhanced chemical vapor deposition (PECVD), a layer of silicon dioxide,phospho-silicate glass (PSG) or boro-phospho-silicate glass (BPSG),commonly 100 to 1000 nanometers thick deposited by PECVD, commonlyleveled by a chemical-mechanical polish (CMP) process, and an optionalPMD cap layer, commonly 10 to 100 nanometers of a hard material such assilicon nitride, silicon carbide nitride or silicon carbide is formed ontop surfaces of the first and second sets of field oxide elements (118,120), the p-type anode contact region (124) and the n-type cathodecontact regions (126).

Still referring to FIG. 1H, an anode contact (130) and cathode contacts(132) are formed in the PMD (128) to make electrical connections to thep-type anode contact region (124) and the n-type cathode contact regions(126), respectively. The contacts (130, 132) are typically formed bydefining contact regions on a top surface of the PMD (128) with acontact photoresist pattern, not shown in FIG. 1H for clarity, removingPMD material from the contact regions using known etching methods toexpose the p-type anode contact region (124) and the n-type cathodecontact regions (126), and filling the contact regions with contactmetal, typically tungsten. Contacts are not formed on the uncontactedn-type diffused ring region (108), leaving the uncontacted n-typediffused ring region (108) free of electrical connections to othercomponents in the IC (100).

During operation of the high voltage diode formed according to theinstant embodiment, holes are injected from the shallow p-well anoderegion (122) into the deep n-well cathode (116). The higher averagedopant density in the uncontacted n-type diffused ring region (108)compared to the average dopant density in the deep n-well cathode (116),combined with placement of the n-type cathode contact regions (126) overthe deep n-well cathode (116) results in holes being desirably reflectedfrom the uncontacted n-type diffused ring region (108) back into thedeep n-well cathode (116), where the holes recombine with electrons orare collected at the n-type cathode contact regions (126). Theconfiguration of the n-sinker regions (108) surrounding the deep n-wellcathode (116) desirably reduces hole current from the shallow p-wellanode region (122) injected into the p-type epitaxial layer (104) andsubstrate (102).

FIG. 2 is a cross-section of an IC containing a high voltage diode withmultiple anode fingers formed according to a second embodiment of theinstant invention. The IC (200) is formed on a p-type substrate (202)with the properties described above in reference to FIG. 1A. A p-typeepitaxial layer (204) with the properties described above in referenceto FIG. 1A is formed on a top surface of the substrate (202). An n-typeburied layer (206) is formed at an interface between the substrate (202)and the p-type epitaxial layer (204) in a region for the inventive highvoltage diode, as described above in reference to FIG. 1A. Anuncontacted n-type diffused ring region (208), extending from a topsurface of the p-type epitaxial layer (204) to the n-type buried layer(206), are formed as described above in reference to FIG. 1B, so as tolaterally surround and electrically isolate a region of the p-typeepitaxial layer (204) over the n-type buried layer (206). A deep n-wellcathode (210) is formed in the p-type epitaxial layer (204) above then-type buried layer (206), as described above in reference to FIG. 1Cand FIG. 1D. An average dopant density in the uncontacted n-typediffused ring region (208) is 100 to 10,000 times higher than theaverage dopant density in the deep n-well cathode (210). A first set offield oxide elements (212) is formed over the deep n-well cathode (210)to electrically isolate anode regions from cathode regions of the highvoltage diode at the top surface of the deep n-well cathode (210). Anoptional second set of field oxide elements (214) may be formed in acontiguous ring over an outer region of the deep n-well cathode (210),as described above in reference to FIG. 1E. Shallow p-well anodes (216)are formed under openings in the first set of field oxide elements(212), as described above in reference to FIG. 1F. P-type anode contactregions (218) are formed at top surfaces of the shallow p-well anodes(216), as described above in reference to FIG. 1G. N-type cathodecontact regions (220) are formed at the top surface of the deep n-wellcathode (210) in openings in the first set of field oxide elements(212), as described above in reference to FIG. 1G. Lateral separationsbetween the shallow p-well anodes (216) and adjacent n-type cathodecontact regions (220) are typically established by a maximum operatingvoltage of the high voltage diode, as discussed above in reference toFIG. 1G. Minimum lateral separations between n-type cathode contactregions (220) and the uncontacted n-type diffused ring region (208) ispreferably greater than a diffusion length of holes in the deep n-wellcathode (210), as discussed above in reference to FIG. 1G. A PMD layer(222) is formed on top surfaces of the first and second sets of fieldoxide elements (212, 214), the p-type anode contact region (218) and then-type cathode contact regions (220), as described above in reference toFIG. 1H. Anode contacts (224) and cathode contacts (226) are formed inthe PMD (222) to make electrical connections to the p-type anode contactregions (218) and the n-type cathode contact regions (220),respectively, as described above in reference to FIG. 1H.

During operation of the high voltage diode formed according to theinstant embodiment, holes are injected from the shallow p-well anoderegions (216) into the deep n-well cathode (210). The higher averagedopant density in the uncontacted n-type diffused ring region (208)compared to the average dopant density in the deep n-well cathode (210),combined with placement of the n-type cathode contact regions (220) overthe deep n-well cathode (210) results in holes being desirably reflectedfrom the uncontacted n-type diffused ring region (208) back into thedeep n-well cathode (210), where the holes recombine with electrons orare collected at the n-type cathode contact regions (220). Theconfiguration of the uncontacted n-type diffused ring region (208)surrounding the deep n-well cathode (210) desirably reduces hole currentfrom the shallow p-well anode regions (216) injected into the p-typeepitaxial layer (204) and substrate (202). The configuration of multipleshallow p-well anode regions (216) in the deep n-well cathode (210)desirably increases a current capacity of the high voltage diode formedaccording to the instant embodiment.

What is claimed is:
 1. A high voltage diode in an integrated circuit,comprising an uncontacted n-type diffused ring region surrounding a deepn-well cathode, wherein: an average dopant density of said uncontactedn-type diffused ring region is higher than an average dopant density ofsaid deep n-well cathode; cathode contacts are formed on said deepn-well cathode; and said uncontacted n-type diffused ring region is freeof electrical connections to other components in said integratedcircuit.
 2. The high voltage diode of claim 1, further comprising ann-type buried layer under, and in contact with, said deep n-wellcathode, wherein an average dopant density of said n-type buried layeris higher than an average dopant density of said deep n-well cathode. 3.The high voltage diode of claim 2, wherein said average dopant densityof said uncontacted n-type diffused ring region is between 100 and10,000 times average dopant density of said deep n-well cathode.
 4. Thehigh voltage diode of claim 3, wherein a lateral separation between saiduncontacted n-type diffused ring region and an n-type contact region ofsaid deep n-well cathode is greater than 5 microns.
 5. The high voltagediode of claim 4, wherein an anode is comprised of a plurality ofseparate p-type wells.
 6. An integrated circuit, comprising a highvoltage diode further comprising: a deep n-well cathode; and anuncontacted n-type diffused ring region surrounding said deep n-wellcathode, wherein: an average dopant density of said uncontacted n-typediffused ring region is higher than an average dopant density of saiddeep n-well cathode; cathode contacts are formed on said deep n-wellcathode; said uncontacted n-type diffused ring region is free ofelectrical connections to other components in said integrated circuit.7. The integrated circuit of claim 6, further comprising an n-typeburied layer under, and in contact with, said deep n-well cathode,wherein an average dopant density of said n-type buried layer is higherthan an average dopant density of said deep n-well cathode.
 8. Theintegrated circuit of claim 7, wherein said average dopant density ofsaid uncontacted n-type diffused ring region is between 100 and 10,000times average dopant density of said deep n-well cathode.
 9. Theintegrated circuit of claim 8, wherein a lateral separation between saiduncontacted n-type diffused ring region and an n-type contact region ofsaid deep n-well cathode is greater than 5 microns.
 10. The integratedcircuit of claim 9, further comprising: a p-well anode formed in saiddeep n-well cathode; a pre-metal dielectric layer formed on a topsurface of said deep n-well cathode; an anode contact formed in saidpre-metal dielectric layer and connected to a p-type anode contactregion formed in said p-well anode; and n-type cathode contact regionsformed in said deep n-well cathode, such that said cathode contacts areconnected to said cathode contact regions.
 11. The integrated circuit ofclaim 10, wherein said p-well anode is comprised of a plurality ofseparate p-type wells in said deep n-well cathode.